System for preventing decomposition of silicon carbide articles during sintering

ABSTRACT

A system to prevent, retard or reverse the decomposition of silicon carbide articles during high temperature plasma sintering. Preferably, the system comprises sintering a silicon carbide refractory or ceramic green body in a closed sintering environment, such as a covered crucible, with strategic placement of the plasma torch or torches, exhaust outlet and crucibles. As sintering proceeds, a silicon vapor pressure builds up within the crucible, retarding the decomposition of the silicon carbide body. The plasma torch, exhaust outlet, and crucibles are positioned so that buoyant convective flow is maximized to increase the heat transfer and energy efficiency. In another embodiment, a &#34;sacrificial&#34; source of silicon carbide is placed into the sintering furnace. The silicon carbide in the sacrificial source starts to decompose before the silicon carbide refractory or ceramic article, creating a supersaturated atmosphere of silicon vapor species in the furnace. This prevents, retards or reverses the silicon carbide decomposition reactions and thus maintains the integrity of the refractory or ceramic article being sintered. Preferably, the sacrificial source is placed in a closed sintering environment along with the silicon carbide article being sintered.

BACKGROUND OF THE INVENTION

This invention relates to a system for preventing, retarding orreversing the decomposition of silicon carbide refractory or ceramicarticles during high temperature sintering.

Silicon carbide has several physical and chemical properties which makeit an excellent material for high temperature, structural uses.Mechanically, silicon carbide is a hard, rigid, brittle solid which doesnot yield to applied stresses even at temperatures approaching itsdecomposition temperature. Because of its high thermal conductivity,silicon carbide is an excellent material for heat exchangers, muffletype furnaces, crucibles, gas-turbine engines and retorts in thecarbothermic production and distillation of zinc. Silicon carbide isalso used in electrical resistance elements, ceramic tiles, boilers,around tapping holes, in heat treating, annealing and forging furnaces,in gas producers, and in other places where strength at hightemperatures, shock resistance and slag resistance are required.Properties associated with silicon carbide refractory and ceramicmaterials are superior strength, high elastic modulus, high fracturetoughness, corrosion resistance, abrasion resistance, thermal shockresistance, and low specific gravity.

Silicon carbide refractory or ceramic materials are generally sinteredat temperatures above 1900° C. so that the silicon carbide articles willdevelop desirable physical and chemical properties such as highstrength, high density and low chemical reactivity. A reducing or inertatmosphere is generally used for sintering silicon carbide to preventformation of compounds which may have undesirable physical or chemicalproperties. Electric kilns are typically used to sinter silicon carbideceramic or refractory materials under controlled atmospheres, but thesetend to be energy inefficient and slow. In the case of a kiln equippedwith graphite heating elements, the voltage can be controlled and thekiln can be heated to fairly high temperatures, yet there are severaldisadvantages: (1) The heating elements have a limited size, complexshape and must be kept under a strictly controlled atmosphere tomaintain a long life; and (2) Furnace size is limited and it isdifficult to achieve a uniform temperature in this type of kiln becausethe heating elements provide only radiant heat. Because of radiant heattransfer, as well as a size limit for heating elements, the kiln has apoor load density, limited productivity, and a poor energy efficiency.

Plasma arc technology has recently been applied to the production ofrefractory and ceramic materials to reduce the furnace energyrequirements and retention times. However, plasma technology hasgenerally only been used for the fusion of high temperature materialsand not for sintering or reaction sintering. This is because therequired sintering temperature for most ceramic or refractory materialsis usually less than 2500° C., whereas the average temperature of gasesheated through a plasma arc column is above about 4000° C. For instance,alpha silicon carbide is generally sintered at temperatures of between1900° C. to 2350° C. At temperatures above around 2150° C., siliconcarbide decomposes into silicon gas and solid carbon. The carbon maythen react further with the silicon carbide and silicon gas to formother vapor species, such as SiC₂ and Si₂ C. This decomposition ofsilicon carbide could result in substantial shrinkage of the articlebeing fired, as well as an undesirable change in surface chemistry.

Plasma arc fired gases differ greatly from ordinary furnace heated gasesin that they become ionized and contain electrically charged particlescapable of transferring electricity and heat; or, as in the case ofnitrogen, become dissociated and highly reactive. For example, nitrogenplasma gas dissociates into a highly reactive mixture of N₂ -molecules,N-atoms, N⁺ -ions and electrons. This dissociation or ionization greatlyincreases the reaction rates for sintering ceramic or refractorymaterials. Nitrogen, for example, which dissociates at around 5000° C.and 1 atmosphere pressure, would not dissociate under the normal furnacesintering conditions of around 1500° C.-2000° C. Thus, the use of plasmagases results in a highly reactive environment, which greatly increasesthe reaction sintering rate.

However, this highly reactive plasma environment also increases thedecomposition of the green body because of the buoyant forces involvedin convective heat transfer which increase the flow of the gases in thefurnace. These gases sweep away the decomposition products, allowing thedecomposition reactions to proceed. In the case of silicon carbide,silicon is continually stripped from the surface of the green body,resulting in a decreased density and undesirable surface chemistry.

SUMMARY OF THE INVENTION

This invention relates to a system for the high temperature sintering ofsilicon carbide refractory or ceramic articles which prevents, retardsor reverses decomposition of the silicon carbide articles.

In a preferred embodiment of the invention, sintering is performed in afurnace with at least one plasma torch positioned near the top of thefurnace, and an exhaust outlet positioned near the bottom of thefurnace. In large furnaces, it is preferable to position an additionaltorch (or torches) through the center of the furnace wall opposite theprimary plasma torch or torches. This specific positioning of the plasmatorch or torches and exhaust outlet serves two functions: (1) Itprovides maximum turbulence within the furnace for convective heattransfer and uniformity of heating, and (2) It prevents the siliconcarbide decomposition reaction products from being swept away from theproximity of the silicon carbide articles being sintered, thus retardingor preventing further decomposition of the silicon carbide articles.

Preferably, the silicon carbide refractory or ceramic articles beingsintered are placed into a crucible, and most preferably, into a coveredcrucible. When crucibles are employed, the silicon carbide decompositionreaction products are contained within the crucibles, thereby preventingfurther the decomposition of the silicon carbide articles. Preferably,the crucibles are made from graphite, but any crucible material, commonto the art, may be used.

In another embodiment of the invention, a sacrificial source of siliconcarbide is placed into the sintering furnace, preferably near the plasmagas inlet. Because of its higher surface area and a close proximity tothe plasma gas inlet, the sacrificial source begins to decompose beforethe refractory or ceramic article being sintered, thereby saturating thefurnace with silicon vapor species. Presence of these gases tends toreverse the silicon carbide decomposition reactions, thus preventing thesilicon carbide refractory or ceramic article from decomposing. Verysmall particles of silicon carbide are preferable for use as thesacrificial source because of their high surface area. Use of asacrificial source in accordance with the present invention results in asuperior sintered product because there is little or no decomposition ofthe silicon carbide refractory or ceramic article.

Accordingly, it is an object of the present invention to provide aneasy, efficient and inexpensive system for the prevention, retardation,or reversal of the decomposition of silicon carbide articles duringplasma sintering.

It is a further object of the present invention to provide a system forpreventing, retarding or reversing the decomposition of silicon carbiderefractory or ceramic articles during sintering which results in highdensity products with low shrinkage.

Other objects and further scope of applicability of the presentinvention will become apparent from the detailed description to follow,taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The sole figure of the drawing is an illustration of the heat transfermechanisms, and positioning of plasma torches, the exhaust outlet andcrucibles, in a plasma arc furnace in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

At the outset, the invention is described in its broadest overallaspects, with a more detailed description following. This inventionrelates to a system for preventing, retarding, or reversing thedecomposition of silicon carbide refractory or ceramic green bodies athigh sintering temperatures. The system comprises sintering a siliconcarbide refractory or ceramic article in a plasma furnace by either: (1)Placing the silicon carbide article in a closed environment, such as acovered crucible; (2) Placing the silicon carbide article in a closedenvironment, such as a covered crucible, along with a sacrificial sourceof silicon carbide; (3) Placing the silicon carbide article in an opencrucible; (4) Placing the silicon carbide article in an open cruciblealong with a sacrificial source of silicon carbide; (5) Placing thesilicon carbide article in a completely open sintering environment alongwith a sacrificial source of silicon carbide; or (6) Sintering thesilicon carbide article in a furnace designed to maximize buoyantconvective heat transfer while minimizing decomposition of the siliconcarbide article, with or without crucibles or a sacrificial source ofsilicon carbide. Preferably, sintering is performed in a furnace with atleast one plasma torch positioned near the top of the furnace, anexhaust outlet positioned near the bottom of the furnace, and a spacebetween crucibles. The specific positioning of the plasma torch, exhaustoutlet, and crucibles, and the presence of the sacrificial source alltend to prevent decomposition of the silicon carbide ceramic orrefractory article.

When silicon carbide is sintered at temperatures above around 2150° C.,it decomposes according to the following reaction:

    SiC.sub.(s) →Si.sub.(g) +C.sub.(s)

This decomposition causes formation of a carbon layer on the siliconcarbide surface, which may then react further with the silicon carbideand silicon gas to form other vapor species, such as SiC₂ and Si₂ C.These reactions result in a net mass loss and decreased density, inaddition to a carbon layer on the surface which may give the siliconcarbide product undesirable chemical and physical properties. Of course,if the environment around the SiC.sub.(s) is saturated with Si.sub.(g),the reaction tends to proceed to the left, that is towards SiC.sub.(s).

The present invention prevents, retards or reverses the decomposition ofsilicon carbide refractory or ceramic green bodies in three ways: (1) Byplacing the silicon carbide green body in a closed environment, such asa covered crucible, a partial pressure of silicon gas builds up withinthe closed environment. As the environment becomes supersaturated withsilicon gas and other silicon vapor species, the decomposition of thesilicon carbide green body is halted or retarded. (2) By positioning theplasma arc torch near the top of the furnace and positioning the exhaustoutlet near the bottom of the furnace, the buoyant effects of convectiveheat transfer are minimized. Thus, the silicon vapor species are notswept away, but remain in the crucibles or the furnace, itself, keepingthe crucible and furnace environment supersaturated with silicon vaporspecies. (3) By employing a sacrificial source of silicon carbon whichdecomposes before the silicon carbide green body, the furnace atmosphere(open or closed environment) becomes saturated with silicon gas andother silicon vapor species, thereby halting, retarding, preventing orreversing the decomposition of the silicon carbide green body.

The preferred starting material for the silicon carbide refractory orceramic green bodies should be primarily alpha, non-cubic crystallinesilicon carbide, since it is more readily available than beta siliconcarbide. It is acceptable, however, to use alpha, beta, or amorphoussilicon carbide or mixtures thereof. If beta silicon carbide isutilized, it should be of high purity. Boron, carbon or carbonizableorganic materials, binders and other additives may be included in thegreen body mixture depending on product requirements.

The silicon carbide starting mixture may be shaped or formed into shapedgreen bodies by any conventional method, such as extrusion, injectionmolding, transfer molding, casting, cold pressing, isostatic pressing orby compression.

The shaped silicon carbide bodies are then sintered in a plasma arcfurnace. The drawing is an illustration of the heat transfer mechanismsinvolved in a plasma arc sintering furnace useful for practicing thisinvention. The straight arrows 24 indicate heat transfer throughradiation and the wavy arrows 26 indicate heat transfer throughconvection. With convective heat transfer, the plasma arc furnace has alower cycle time than prior art furnaces.

Copending patent applications, Ser. No. 533,596, filed Sept. 19, 1983,now U.S. Pat. No. 4,559,312, entitled PLASMA HEATED SINTERING FURNACE,and Ser. No. 718,375 filed 4-1-875 entitled PLASMA ARC SINTERING OFSILICON CARBIDE, to Jonathan J. Kim et al, filed on even date herewith,the teachings of which are incorporated herein by reference, are usefulin practicing the present invention. U.S. Ser. No. 533,596, filed9-19-83 now U.S. Pat. No. 4,559,312 discloses a plasma heated furnaceand method for sintering refractory or ceramic materials. In a preferredembodiment, the furnace comprises at least two plasma torch inlets,positioned asymmetrically through the walls of the sintering chamber,with one plasma torch inlet positioned near the top of the sinteringchamber, the other plasma torch inlet positioned near the center of thefurnace, and the exhaust outlet positioned near the bottom of thesintering chamber Ser. No. 718,375, filed 4-1-85 discloses a process forthe sintering of silicon carbide refractory or ceramic articles in aplasma heated furnace, wherein the silicon carbide article is heated bya plasma gas having an energy capacity of 2000 BTU/lb-6000 BTU/lb to asintering temperature of between 1500° C.-2500° C., at a heating rate of300° C./hr-2000° C./hr, and held at the sintering temperature for 0.1-2hours. A typical cycle time for the present invention operating inaccordance with Ser. No. 533,596, filed 9-19-83, now U.S. Pat. No.4,559,312, and Ser. No. 718,375 is around eight hours (includingcooling), which compares to a total cycle time of around 24 hours for anelectric kiln. It should be noted that in prior art electric kilns, suchas a Centorr™ or Astro™ furnace, the only mode of heat transfer isthrough radiation.

A plasma sintering furnace is a turbulent flow system, unlike prior artradiant furnaces which are considered stagnant systems. Buoyantconvective forces in a turbulent flow system increase the heat transferrate and provide uniformity of heating.

It is preferable to sinter the silicon carbide green bodies in a closedenvironment, such as a covered crucible, to prevent the decompositionproducts from being swept away and thereby preventing, retarding orreversing the silicon carbide decomposition reaction. In such anembodiment, as illustrated in the drawing, the silicon carbide shapedgreen bodies 10 (shown as rotors in the drawing) are preferably placedinto a closed environment, such as a covered crucible 12. As sinteringproceeds, a partial pressure of silicon is built up within the crucible.(Thermodynamic calculations have shown that at 2325° C., the partialpressure of silicon in equilibrium with silicon carbide is 3×10⁻³ atm.)As the silicon vapors build up, the decomposition of the silicon carbideis halted, retarded or reversed.

A space 14 should be allowed between the crucibles 12 to increase theavailable surface area for heat transfer through convection. The spacesenable flow of the furnace gases and thus better convective heattransfer. Crucibles may be stacked, as shown in the drawing. The space14 between crucibles should be around at least 0.5 inches. A preferredcrucible material is graphite, although any crucible material, common tothe art, may be utilized.

The location of the plasma torch in the furnace is very important toobtain maximum turbulence, and thereby maximize heat extraction, obtaina high energy efficiency, and minimize temperature gradients in thefurnace to obtain uniform sintering and thus consistent products. Theplasma torch 16 is preferably positioned near the top of the furnace, asshown in the drawing, to avoid cold pockets. In large furnaces, anadditional torch or torches 18 are preferably positioned through thecenter of the furnace wall opposite the primary torch 16, as is shown inthe drawing. Preferably, the exhaust outlet 20 is positioned near thebottom of the furnace to minimize heat losses.

In another embodiment, the system of the present invention forpreventing the decomposition of silicon carbide comprises theutilization of a sacrificial source of silicon carbide, which decomposesbefore the silicon carbide refractory or ceramic article being sintered.The decomposition products from the sacrificial source saturate thefurnace environment with silicon gas and other vapor species, whichreverse, prevent or retard the silicon carbide refractory or ceramicarticle from decomposing. Preferably, a closed environment is used, suchas a covered crucible, in conjunction with a sacrificial source, whereinthe sacrificial source is placed inside the crucible with the shapedgreen refractory or ceramic body. An open environment may also be used,wherein the shaped green bodies are placed into open crucibles or placedopenly into the furnace along with a sacrificial source of siliconcarbide.

Normally during sintering of alpha silicon carbide, or other ceramicarticles, densification occurs through shrinkage; material from one areaof the article is transported to another region of the same article.Usually, material is not exchanged between objects undergoing sintering.The mechanism of densification, important for sintering alpha siliconcarbide using the system of the present invention, involves sublimationand condensation. In this mechanism, there is sublimation of thesacrificial source (source) and condensation on the article beingsintered (sink). In order to achieve this preferential transport ofmass, the chemical potential of the source must be higher than that ofthe sink.

A higher chemical potential and a transport of mass from the sacrificialsource to the silicon carbide green bodies may be achieved in threemanners: (1) By holding the source at a higher temperature than thesink. This can be effected by placing the sacrificial source in thefurnace near the plasma gas inlet; (2) By making the source from ameta-stable form of material, such as a meta-stable crystal phase or anamorphous structure; or (3) By providing source particles having a verysmall radius of curvature as compared to the particles of the shapedgreen bodies.

Preferably, the sacrificial source should have a much higher surfacearea than the refractory article being sintered so that the sacrificialsource decomposes at a faster rate. Small particles of silicon carbideare preferable since they have both a small radius of curvature and ahigh surface area. The particles should be spread out as thinly aspossible to increase the available surface area and to minimizesintering of the sacrificial source which would lower the availablesurface area and increase the radius of curvature of the individualparticles.

Preferably, the sacrificial source or particles are placed near thesilicon carbide article to be sintered in a crucible, and mostpreferably in a covered crucible. A slurry of silicon carbide sourceparticles may be used to coat the insides of the crucible.Alternatively, the article to be sintered may be coated with siliconcarbide powder. As another alternative, silicon carbide powder may beplaced as a layer in the bottom of the crucible.

In another embodiment, the shaped silicon carbide green bodies arestacked or placed openly in the furnace. The silicon carbide sourceparticles may be placed openly in the furnace near the silicon carbidearticles, or in a large shallow container to maximize the availablesurface area. In a completely open furnace environment, it is preferableto locate the sacrificial source material near the plasma gas inlet sothat it will be at a higher temperature and thus decompose more quicklythan the silicon carbide refractory or ceramic articles. Alternatively,a slurry containing the silicon carbide particles may be applied to thefurnace walls.

It is preferable to use an oxygen-free gas for sintering silicon carbidearticles, so that oxides will not be produced which may have undesirablephysical and chemical properties. The preferred gases for sintering ofsilicon carbide are nitrogen, argon, helium, and/or neon, however, anyplasma gas may be used in accordance with the present invention,depending upon product requirements.

The system of this invention is useful for sintering standard refractoryor ceramic silicon carbide shapes or complex shapes such as backplates,rotors, scroll assemblies and nozzles. Use of the system of thisinvention results in a product with a good density and a gooddimensional tolerance.

The invention is further illustrated by the following non-limitingexamples.

EXAMPLES 1-20

Shaped green bodies were positioned at varying heights in an opensintering arrangement within a plasma fired furnace to determine theirweight loss and density as a function of furnace position. The plasmatorch was located near the top of the furnace and the exhaust outlet waslocated near the bottom of the furnace. The samples were heated at 300°C./hr to 2325° C. and held for 1 1/2 hours at 2325° C. The weight ofeach green body was measured before and after sintering to determine theweight loss. Density measurements were taken of the sintered bodies.Results are given in Table 1. Weight loss and density results for a castsilicon carbide body, extending the entire furnace height, are alsogiven in Table 1 for comparison.

                  TABLE 1                                                         ______________________________________                                        Summary of Open Sintering                                                     Ex-   Inches From                                                                              Green   Sintered                                                                             % of    Density                               ample Bottom     Wt (g)  Wt (g) Green Wt                                                                              (g/cm.sup.3)                          ______________________________________                                        Cast  Furnace Ht.                                                                              1580    1320   83.5    2.95                                   1    0.0        5.418   4.268  78.8    2.52                                   2    0.0        5.405   4.549  84.2    2.801                                  3    0.0        5.444   4.613  84.7    2.821                                  4    0.0        5.433   4.616  85.0    2.795                                  5    0.0        5.395   4.504  83.5    2.774                                  6    0.0        5.450   4.648  85.3    2.852                                  7    0.0        5.411   4.663  86.1    2.875                                  8    0.0        5.415   4.508  83.3    2.763                                  9    0.0        5.382   4.449  82.7    2.729                                 10    0.0        5.392   4.492  83.3    2.765                                 11    13.375     5.438   4.398  80.9    2.710                                 12    13.375     5.408   4.476  82.8    2.770                                 13    13.375     5.408   4.422  81.8    2.739                                 14    13.375     5.394   3.625  67.2    2.669                                 15    13.375     5.340   4.250  79.5    2.648                                 16    13.375     5.436   4.08   75.0    2.622                                 17    14.25      5.411   4.237  78.3    2.626                                 18    14.375     5.415   4.689  86.6    2.820                                 19    14.375     5.425   4.754  87.6    2.886                                 20    16.25      5.438   4.245  78.1    2.618                                 ______________________________________                                    

Table 1 shows that, in general, the green bodies positioned nearer thetop of the furnace (where the plasma torch was positioned) had a lowerdensity and a higher weight loss than samples positioned towards thebottom of the furnace.

EXAMPLES 21-25

Density measurements were taken of silicon carbide bodies after beingsintered in an open sintering environment to compare the effects offurnace position and a sacrificial source of silicon carbide on density.The green bodies were placed at the top or bottom of a plasma firedfurnace with the plasma torch located near the top of the furnace andthe exhaust outlet located near the bottom of the furnace. A sacrificialsource of silicon carbide was present in some of the runs. One of thesilicon carbide bodies was placed very close to the plasma torch inlet(top impinged). The average densities obtained are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Effects of Furnace Position and Presence of a Sacrificial                     Source on Density in an Open Sintering Environment                                    Location in  Sacrificial                                                                             Average Density                                Example Furnace      Source    (g/cm.sup.3)                                   ______________________________________                                        21      Bottom       No        2.770                                          22      Top          No        2.711                                          23      Bottom       Yes       2.907                                          24      Top          Yes       2.728                                          25      Top Impinged Yes       2.651                                          ______________________________________                                    

Table 2 shows that samples located at the top of the furnace (where theplasma torch was positioned) had a lower density than those located onthe bottom of the furnace. Also, the presence of a sacrificial sourceresulted in higher densities.

EXAMPLES 26-30

Density measurements were taken of silicon carbide bodies after beingsintered within open crucibles to compare the effects of furnaceposition and a sacrificial source on density. The green bodies wereplaced at the top or bottom of a plasma fired furnace with the plasmatorch located near the top of the furnace and the exhaust outlet locatednear the bottom of the furnace. A sacrificial source of silicon carbidewas present in some of the runs. One of the samples was impinged by theplasma torch. The average densities obtained are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Effects of Furnace Position and Presence of a Sacrificial                     Source on Density in an Open Crucible Arrangement                                     Location in  Sacrificial                                                                             Average Density                                Example Furnace      Source    (g/cm.sup.3)                                   ______________________________________                                        26      Bottom       No        3.143                                          27      Top          No        3.109                                          28      Bottom       Yes       3.152                                          29      Top          Yes       3.155                                          30      Top Impinged Yes       2.826                                          ______________________________________                                    

Comparing the results of Table 3 with those of Table 2 demonstrate thatplacing the silicon carbide green bodies in an open crucible yields muchhigher densities than when using a complete open sintering environment.Table 3 also shows higher densities being obtained when a sacrificialsource was present except when the sample was impinged by the plasmatorch.

EXAMPLES 31-36

Density measurements were taken of silicon carbide bodies sintered inseveral runs within an open crucible and containing a silicon carbidesacrificial source, and sintered within a closed crucible not containinga sacrificial source, to compare the effects on density of (1) furnaceposition, (2) a sacrificial source, and (3) covering the crucible. Thegreen bodies were placed in crucibles at the top and bottom of a plasmafired furnace with the plasma torch located near the top of the furnaceand the exhaust outlet located near the bottom of the furnace. Asacrificial source of silicon carbide was placed into the open cruciblesonly. One of the samples was impinged by the plasma torch. The averagedensities obtained are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Effects of Furnace Position, Closed Sintering, and Open                       Sintering With a Sacrificial Source on Density                                Ex-   Location in Sacrificial                                                                             Cruci-                                                                              Average Density                             ample Furnace     Source    ble   (g/cm.sup.3)                                ______________________________________                                        31    Bottom      No        Closed                                                                              3.157                                       32    Top         No        Closed                                                                              3.149                                       33    Bottom      Yes       Open  3.152                                       34    Top         Yes       Open  3.155                                       35    Top Impinged                                                                              No        Closed                                                                              3.140                                       36    Top Impinged                                                                              Yes       Open  3.004                                       ______________________________________                                    

Table 4 shows that impingement of the samples by the plasma torchresults in the lowest densities. Table 4 also shows that positioning ofthe green body in the bottom of the plasma furnace (away from the plasmatorch) results in higher densities. Table 4 also shows that similardensities are obtained using an open crucible containing a sacrificialsource, and using a closed crucible not containing a sacrificial source.

EXAMPLES 37-42

Silicon carbide articles were sintered in three different arrangementsin a plasma furnace: (1) Two covered crucibles, coated on the insidewith a slurry of silicon carbide powder having a surface area of 129 m²/g, one positioned at the top of the furnace and the other positioned atthe bottom of the furnace; (2) Two covered crucibles, containing siliconcarbide powder with a surface area of 36 m² /g, one positioned at thetop of the furnace and the other positioned at the bottom of thefurnace; and (3) Two covered crucibles, containing no sacrificialsource, one positioned at the top of the furnace and the otherpositioned at the bottom of the furnace. A plasma torch was located nearthe top of the furnace and the exhaust outlet was located near thebottom of the furnace. Density measurements were taken of all thesamples. Results are given in Table 5.

                  TABLE 5                                                         ______________________________________                                        Effect of Sacrificial Source on Closed Sintering Environment                            Location in   Type of  Density                                      Example   Furnace       Source   (g/cm.sup.3)                                 ______________________________________                                        37        Bottom        None     3.137                                        38        Bottom        Coating  3.143                                        39        Bottom        Powder   3.128                                        40        Top           Coating  3.135                                        41        Top Impinged  Powder   3.089                                        42        Top Impinged  None     3.116                                        ______________________________________                                    

Table 5 shows that samples located at the bottom of the furnace had ahigher density than those located at the top of the furnace (where theplasma torch was positioned). The lowest densities were obtained whenthey were impinged by the plasma torch. Use of a coating as thesacrificial source, in general, resulted in higher densities.

Accordingly, a system has been provided for preventing, retarding orreversing the decomposition of silicon carbide articles during hightemperature plasma sintering. The system comprises, in one embodiment,positioning of the plasma torch near the top of the furnace and theexhaust outlet near the bottom of the furnace, leaving a space betweencrucibles and covering the crucibles. In another embodiment, the systemcomprises the use of a sacrificial source of silicon carbide whichdecomposes before the silicon carbide refractory or ceramic article.

Although the invention has been described with reference to itspreferred embodiment, other embodiments can achieve the same results.Variations and modifications of the present invention will be obvious tothose skilled in the art and it is intended to cover in the appendedclaims all such modifications and equivalents.

We claim:
 1. A method for preventing, halting, retarding or reversingthe decomposition of silicon carbide articles during high temperaturesintering in a plasma furnace comprising:placing formed silicon carbidearticles into at least one covered crucible within a plasma furnace; andsintering said silicon carbide articles in a plasma gas environment sothat a partial pressure of silicon gas builds up within the closedenvironment rendered by the covered crucible, thereby saturating theenvironment with silicon gas and other silicon vapor species, haltingfurther decomposition of said silicon carbide articles.
 2. A method inaccordance with claim 1 wherein said crucible is made from graphite. 3.A method in accordance with claim 1 comprising at least two stackedcrucibles, wherein a space of at least 0.5 inches is provided betweensaid crucibles.
 4. A method in accordance with claim 1 furthercomprising positioning at least one primary plasma torch near the top ofsaid furnace.
 5. A method in accordance with claim 4 further comprisingpositioning at least one additional plasma torch through the center wallof said furnace and opposite said primary plasma torch.
 6. A method inaccordance with claim 4 further comprising positioning an exhaust outletnear the bottom of said furnace.
 7. A method in accordance with claim 1further comprising placing a sacrificial source of silicon carbide insaid furnace, said sacrificial source of silicon carbide beginningdecomposition before the silicon carbide articles being sintered.
 8. Amethod in accordance with claim 7 wherein said sacrificial source ofsilicon carbide has a higher surface area than the particles of thesilicon carbide articles being sintered.
 9. A method in accordance withclaim 7 wherein said sacrificial source of silicon carbide has a smallerradius of curvature than the particles of the silicon carbide articlesbeing sintered.
 10. A method in accordance with claim 7 wherein saidsacrificial source of silicon carbide is in the form of a meta-stablematerial.
 11. A method in accordance with claim 7 wherein saidsacrificial source of silicon carbide is placed into said crucible. 12.A method in accordance with claim 11 wherein said sacrificial source ofsilicon carbide comprises a coating on the inside of said crucible. 13.A method in accordance with claim 11 wherein said sacrificial source ofsilicon carbide comprises a powder on the bottom of said crucible.
 14. Amethod in accordance with claim 7 wherein said sacrificial source ofsilicon carbide is placed into a shallow container.
 15. A method inaccordance with claim 7 wherein said sacrificial source of siliconcarbide is placed near at least one plasma torch inlet.
 16. A method forpreventing, halting. retarding or reversing the decomposition of siliconcarbide articles during high temperature sintering in a plasma furnacecomprising:placing formed silicon carbide articles into a plasmafurnace; injecting a plasma arc heated gas from at least one primarytorch which is positioned near the top of said furnace in order to avoidthe formation of cold pockets; sintering said silicon carbide articlesin a plasma gas environment produced by said primary plasma torch; andexpending the exhaust produced by said plasma gas through an exhaustoutlet positioned near the bottom of said furnace in order to minimizethe buoyant effects of the convected heat transfer resulting therefrom.17. A method in accordance with claim 16 further comprising positioningat least one additional plasma torch through the center wall of saidfurnace and opposite said primary plasma torch.
 18. A method inaccordance with claim 16 further comprising placing a sacrificial sourceof silicon carbide in said furnace, said sacrificial source of siliconcarbide beginning decomposition before the silicon carbide articlesbeing sintered.
 19. A method in accordance with claim 18 wherein saidsacrificial source of silicon carbide has a higher surface area than theparticles of the silicon carbide articles being sintered.
 20. A methodin accordance with claim 18 wherein said sacrificial source of siliconcarbide has a smaller radius of curvature than the particles of thesilicon carbide articles being sintered.
 21. A method in accordance withclaim 18 wherein said sacrificial source of silicon carbide is in theform of a meta-stable material.
 22. A method in accordance with claim 18wherein said sacrificial source of silicon carbide is placed into ashallow container.
 23. A method in accordance with claim 18 wherein saidsacrificial source of silicon carbide is placed near at least one plasmagas inlet.
 24. A method in accordance with claim 18 wherein saidsacrificial source of silicon carbide comprises a coating on the furnacewalls.
 25. A method for preventing, halting, retarding or reversing thedecomposition of silicon carbide articles during high temperaturesintering in a plasma furnace comprising:placing formed silicon carbidearticles into a plasma furnace; placing a sacrificial source of siliconcarbide in said plasma furnace, said sacrificial source of siliconcarbide beginning decomposition before the silicon carbide articlesduring sintering; and; sintering said silicon carbide articles in aplasma gas environment.
 26. A method in accordance with claim 25 whereinsaid sacrificial source of silicon carbide has a higher surface areathan the particles of the silicon carbide articles being sintered.
 27. Amethod in accordance with claim 25 wherein said sacrificial source ofsilicon carbide has a smaller radius of curvature than the particles ofthe silicon carbide articles being sintered.
 28. A method in accordancewith claim 25 wherein said sacrificial source of silicon carbide is inthe form of a meta-stable material.
 29. A method in accordance withclaim 25 wherein said sacrificial source of silicon carbide is placedinto a shallow container.
 30. A method in accordance with claim 25wherein said sacrificial source of silicon carbide is placed near atleast one plasma torch inlet.
 31. A method in accordance with claim 25wherein said sacrificial source of silicon carbide comprises a coatingon the furnace walls.
 32. A furnace for the high temperature sinteringof silicon carbide articles and for preventing, halting, retarding orreversing the decomposition of said silicon carbide articles duringsintering, comprising:a sintering chamber; at least one covered cruciblefor containing said silicon carbide articles within said sinteringchamber; a plasma gas inlet positioned near the top of said sinteringchamber; and, an exhaust outlet positioned near the bottom of saidsintering chamber.
 33. A furnace in accordance with claim 32 whereinsaid sacrificial source is placed into said crucible.
 34. A furnace inaccordance with claim 32 wherein there are at least two crucibles whichare stacked in a vertical direction within said sintering chamber, witha space of at least 0.5 inches between said crucibles.
 35. A furnace forthe high temperature sintering of silicon carbide articles and forpreventing, halting, retarding or reversing the decomposition of saidsilicon carbide articles during sintering, comprising:a sinteringchamber; at least one crucible for containing said silicon carbidearticles within said sintering chamber; a plasma gas inlet positionednear the top of said sintering chamber; an exhaust outlet positionednear the bottom of said sintering chamber; and a sacrificial source ofsilicon carbide supported within said sintering chamber, wherein saidsacrificial source of silicon carbide begins decomposition before thesilicon carbide articles being sintered.
 36. A furnace in accordancewith claim 35 wherein said sacrificial source is placed into a shallowcontainer within said sintering chamber.
 37. A furnace in accordancewith claim 35 wherein said sacrificial source is placed into saidcrucible.